1. Field of the Invention
The invention relates to electronic circuits and more particularly to an interface circuit and a method for improving the speed of the circuit interfacing an MOS digital system with a digital system having different signal requirements.
2. Description of the Prior Art
Prior art digital systems can be roughly divided into system utilizing MOS transistors and those utilizing bipolar transistors. The MOS system typically generates digital signals with the difference between a logic "0" and a logic "1" level being several volts. By contrast, the difference in the logic levels of digital systems utilizing bipolar transistors may be as low as a few millivolts. In order to use both MOS and bipolar circuits in the same system, it becomes necessary to provide some means for converting the logic signals generated by MOS digital circuits to those generated by bipolar digital circuits. Typical prior art techniques for converting the logic level generated by MOS digital circuits to those acceptable by bipolar digital circuits was to utilize a grounded source MOS amplifier to supply a voltage signal to one input of a differential comparator. The other input to the comparator was a stable reference voltage. The output of the comparator would be a two level signal compatible with bipolar circuits with the level being determined by whether or not the output signal from the MOS digital circuit exceeded the reference voltage. This technique resulted in a relatively slow circuit due to the inherent frequency limitations of grounded source MOS output transistor.